The last two years of Jimi Hendrix's life were a time of constant change. After releasing a final studio album, 1968's Electric Ladyland, the Jimi Hendrix Experience broke up, and the guitarist faced litigation over a contract he'd signed before he was famous. To fulfill the terms of the agreement, he put out the 1970 live outing Band of Gypsys with a new lineup of musicians. It would be the last LP he'd release before his death later that year, despite having stockpiled stacks of tapes for a new studio album.

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Much of the music he tracked in the late Sixties has come out on posthumous albums, beginning with 1971's The Cry of Love, which contained songs that Hendrix had mixed with his go-to engineer Eddie Kramer. Since the mid-Nineties, Kramer and Hendrix's estate have painstakingly trawled the vaults for previously unreleased gems. They issued the first of three compilations, Valleys of Neptune, in 2010 and followed it up with People, Hell and Angels three years later. Now they've slated a final volume, Both Sides of the Sky – which highlights the many changes in Hendrix's working life as he played with different configurations of musicians between 1968 and 1970 – for a March release.

Beginning with "Mannish Boy," a bluesy rocker that finds Hendrix singing along with his lead guitar lines, the record shows how much fun he was having at the time. In a rendition of "Lover Man," a speedy tune he was fooling around with since at least 1967's Are You Experienced, he interpolates the theme from TV's Batman. And on a country-inflected take on "Stepping Stone," the last single he released in his lifetime, he plays catcalls and schoolyard taunts on his guitar in between lyrics.

"Imagine Jimi doing a take and when everything falls apart, he'd start playing the Batman or Peter Gunn theme without missing a beat," Kramer says. "He'd do something really silly and stupid and everybody would be cracking up. He wanted to keep it light. He'd also do it to change it up a bit and inevitably those lines would work themselves into songs, and that's Jimi's sly humor."

Elsewhere, the album features Hendrix's collaborations with Johnny Winter, Lonnie Youngblood and Stephen Stills. With the latter, Hendrix played sideman on an upbeat, organ-saturated, pre–Déjà Vu rendition of Joni Mitchell's "Woodstock." Stills looks back fondly on the day he recorded "Woodstock" with Hendrix. It was late 1969, and the musicians were at the New York studio the Record Plant. At one point, Hendrix picked up a bass and they started playing the song. "I wanted to see what would happen playing that with him," Stills says. "He was just the gentlest guy. Watching him play was like watching the greatest athlete you ever saw, like Julius Irving or Muhammad Ali. It was unbelievable. He taught me to quit thinking and let it happen."

Many of the songs are alternate takes of tunes Hendrix fans already know, but they provide context and new insights into his process in his final years. "I have had the good fortune of being able to listen to everything in the vault and I got the sense that some of these songs trace back to '67 and he was still working on them," co-producer John McDermott says. "Jimi really tried to be whittling down songs, working hard to refine things. There was a constant evolution of the content."

"Jimi taught me to quit thinking and let it happen." –Stephen Stills

At the time of his death, most of the recordings on Both Sides of the Sky were unfinished by Hendrix's standards. "He probably would have revisited it and said, 'I can do that better,'" Kramer says. "He was never satisfied." But he says that doesn't undersell the brilliance of the music on Both Sides. Pointing to "Cherokee Mist" – a feedback-infused instrumental with an elastic melody that features Hendrix on sitar as well as guitar – he says, "At the very end, there's that incredible sound that he's getting out of his amplifier. It just sounds like a beast that got loose in the studio. It's primitive and wonderful."

As Hendrix would make each new recording, it would be logged and stored at his own Electric Lady Studios, which he opened in August 1970. "Electric Lady had this enormous wall of closets, and most of them were full of tapes, and only Jimi and I had the keys," Kramer recalls. "We'd say, 'Let's get this pile out and go through it.' He had these legal pads, and as we listened he would write down precisely what the instruments should be doing." But before they could finish the album, Hendrix died while on tour in Europe that September.

Even though Electric Lady was open for only a short time, Kramer has vivid memories of working on music there with Hendrix. The sessions would be like performances in and of themselves: Hendrix would be handling the faders for the guitar and vocals, and Kramer would be leaping around the console trying to keep it all together. "After we faded down, we'd collapse laughing," he says. It was that experience, both having fun with Hendrix and learning what he wanted from a mix, that has provided the template for how Kramer has mixed all of Hendrix's posthumous releases.

Now Both Sides of the Sky may be the last word on Hendrix in the studio. "It is hard to say if this is it," McDermott says. "With Jimi, there is always hope that there is a cache of tapes somewhere out there that would really be great. God only knows what he would have done if he had lived. He just loved to record and create."